Pop the top cover off and see if grabbing the toroidal power transformer body/top with your hand quells the buzzing (yes, this is safe, unless you've stripped insulation from the transformer windings & leads). If so, check the hold-down bolt (thru transformer center) for tightness and correct (gently) if necessary. Should the buzz remain, call Klaus for further advice; an excursion to Indy may be appropriate.
Back in the "olden" days, mechanical "buzz" of transformer cases was frequently found. Today, with modern components, especially toroidal transformers, it shouldn't "hardly ever" happen, absent some other issue (e.g., loose bolts).
A curious oddity is that the current Stratos uses a "reduced noise" Plitron transformer, yet the Tempest employs a "standard" unit. For us electronics geeks, that's backwards of what we would expect.
